N-octadienol is useful as an intermediate for synthetic resin modifiers, agricultural chemicals, medicines, perfumes, and the like. Particularly, n-octanol which is obtained by reduction of n-octadienol is an important starting material for the preparation of di-n-octyl phthalate which is widely used as a plasticizer for polyvinyl chloride and similar polymers. Di-n-octyl phthalate is superior in use for various basic applications when compared to the conventional dioctyl phthalate derived from 2-ethylhexanol.
It has been proposed that n-octanol can be prepared by reacting butadiene with water in the presence of a palladium catalyst to synthesize n-octadienol followed by hydrogenation of n-octadienol (e.g., U.S. Pat. No. 3,670,032). However, according to the process disclosed in U.S. Pat. No. 3,670,032, both the rate of formation of n-octadienol and the selectivity toward it are extremely low so that the process is not suitable for the commercial production of n-octanol.
Since palladium is an extremely expensive metal, it is essential for the commercial production of n-octadienol by the reaction of butadiene with water to keep the activity of the catalyst stable for a prolonged period of time. In order to stabilize the activity of the catalyst, an addition of the phosphine in excess is necessary, but such excessive addition of the phosphine leads to unsatisfactory results in that, as mentioned above, both the rate of formation of and the selectivity toward n-octadienol are decreased.
Furthermore, in the synthesis of n-octadienol the product is generally isolated by direct distillation from the reaction mixture, and the distillation residue containing the catalyst components is recycled to the reaction system. However, the palladium catalyst has a tendency toward deterioration or metallization at distillation temperatures exceeding about 120.degree. C. The deterioration and metallization of the palladium catalyst are serious problems from a commercial point of view, since these factors not only result in a decrease in the catalyst activity, but they make substantially impossible a continuous reuse of the catalyst. In order to suppress the deterioration and metallization of the palladium catalyst, it is necessary to conduct the distillation of products from the reaction mixture at a temperature of 120.degree. C. or below, but in such cases other problems such as a build-up of high boiling by-products including octadienyl ether in the reaction system and a decrease in the distillation yield of n-octadienol may arise.
Thus, in order to achieve a truly industrial valuable method of n-octadienol synthesis by the reaction of butadiene and water in the presence of a palladium catalyst, it is essential to solve several problems which include the following: (1) the rate and the selectivity of the reaction must be improved to commercially acceptable levels, (2) catalyst life must be maintained over a prolonged period of time, and (3) the product must be isolated from the reaction mixture and the catalyst must be recycled without loss of catalyst activity.
U.S. Pat. No. 4,356,333 (Yoshimura et al.), which issued on Oct. 26, 1982, and which is incorporated herein by reference, attempts to overcome the problems set forth above by a process which comprises: (1) conducting the reaction of butadiene and water in an aqueous sulfolane solution having a water to sulfolane weight ratio of 20:80 to 70:30 and containing carbonate and/or bicarbonate ions in the presence of a palladium or a palladium compound catalyst, a monodentate phosphine ligand, and a monodentate tertiary amine; (2) subjecting the reaction mixture to solvent extraction; and (3) recycling the extraction residue containing the catalyst components to the first step, i.e., the reaction of butadiene and water.
The aqueous sulfolane solution permits easy separation of the product from the reaction mixture by means of extraction. That is, sulfolane is chosen as a co-solvent with water to avoid a reaction mixture that would be homogeneous, thus making catalyst recovery either very difficult or impossible. By the combined use of an aqueous sulfolane solution and a monodentate phosphine the palladium catalyst and sulfolane become substantially insoluble in the extractant and losses of the palladium catalyst, phosphine, tertiary amine and sulfolane because of their dissolution into the extractant layer become negligibly small. The desired product, i.e., n-octadienol, is subsequently separated from the reaction mixture by solvent extraction wherein the octadienol is extracted into the solvent and the catalyst and ligand are left in the aqueous layer. The extraction is generally carried out under an atmosphere of carbon dioxide or an inert gas in an extraction column. In order to remove trace amounts of catalyst and phosphine from the extract layer, the extract is washed with water or aqueous sulfolane. Thereafter, the catalyst is recycled to the synthesis step after being partially re-activated.
Moreover, since the problem of deterioration and metallization of the palladium catalyst due to heat and build-up of high boiling by-products can be overcome by the adoption of an extraction method, the catalyst activity can be kept more stable. Such advantages attributable to the extraction method can not be obtained in the absence of either the aqueous sulfolane solution or the monodentate phosphine. For example, when the ligand is triphenylphosphine substantial portions of the palladium and phosphine are extracted into the extractant layer even if the n-octadienol synthesis is carried out in an aqueous sulfolane solution, and hence the extraction procedure cannot be successfully applied.
The isolation of the octadienol from the extract layer is accomplished by distillation where it is separated from unreacted starting material, by-products and the extraction solvent.
The present inventor has discovered a novel means for simplifying the n-octadienol process of U.S. Pat. No. 4,356,333, and, in turn, substantially reducing the capital cost of the method in terms of system requirements and chemical additions. The present invention utilizes a hydrophobic membrane in a perstraction mode to separate as retentate the ligand and catalyst from a permeate of n-octadienol and unreacted butadiene feed. Thus, the need for extraction columns, distillation columns for extraction solvent separation and recovery from the reaction product, the extraction solvent itself and a catalyst re-activation section are avoided.
The use of a hydrophobic membrane and butadiene as the perstraction solvent allow more optimum synthesis conditions by eliminating the need for a reaction solvent that will separate easily in the extraction column; simplify the catalyst and ligand separation and recycle step; eliminate the extraction step thereby eliminating expensive extraction columns and the need for an extraction solvent and its subsequent wash step; reduce expensive distillation requirements to separate the extraction solvent from reaction products and to further purify it for recycle; and reduce losses of palladium and phosphine.
Although the reaction rate is satisfactory using the sulfolane, the rate would be faster if solvents or surfactants that promote phase transfer of the butadiene to the aqueous layer could be used. However, when phase transfer is promoted, emulsions of the mixture often result and solvent extraction becomes difficult. Membranes may be used to conveniently separate the organic-soluble constituents from the water soluble constituents from such mixtures. Thus, using the present invention, an optimum reaction system may be chosen without regard to the difficulties normally presented by emulsified reaction products.
The present invention also simplifies the catalyst and ligand separation and recycle step. In the conventional process, as discussed above, the catalyst and ligand may be recycled only after the reaction mixture is settled such that the aqueous layer contains the ligand and catalyst, and only after the catalyst has been re-activated. In the present invention, the ligand and catalyst may be recycled directly to the reactor along with any of the butadiene feed that may have permeated the membrane into the aqueous layer. No further treatment should be required.
Since the present invention eliminates the extraction step, expensive extraction columns, an extraction solvent and its subsequent wash step are no longer required. The effect of this is to reduce capital cost for the extraction column and wash column and reduce energy and solvent costs. Other costs eliminated are the capital cost for constructing and operating the distillation towers needed to separate the extraction solvent from the reaction products and to further purify it for recycle.
Finally, it is well known that reduction of process steps reduces the loss of feed, product and other components of the mixture such as, in the present case, expensive palladium and phosphine. This invention substantially reduces the number of steps required to isolate the product and recover and recycle the catalyst thus a reduction of losses is anticipated.
The present invention also provides many additional advantages which shall become apparent as described below.